A mass grave with the remains of hundreds of likely indigenous miners during the Spanish colonial era has been discovered by construction workers in Bolivia, a researcher said Saturday. The construction workers, who have had no specialized excavation training, claim the remains could be of 400 to 500 people, and that there may be many more.

As construction was started on a new building in the "El Minero" district of Potosi this week, the workers found the remains located high up in the Andes, Agence France-Presse reported. "We are talking about a common grave found at about 1.8 meters (5.9 feet), and the human remains are scattered over an area of four by four meters," said Sergio Fidel, a researcher at a museum belonging to Tomas Frias University, adding that the university staff got involved after they learned that the workers were piling the bones in a massive heap, fully exposed as construction continued.

In the Spanish colonial era, Potosi became famous for its massive silver and tin reserves, which started to be mined in the 16th century. Local indigenous people, mainly ethnic Aymara, were commonly put to work as both slaves and indentured servants, especially at the famed Cerro Rico (Rich Hill) mountain.

While one hypothesis is that they happened on an indigenous burial ground of slaves and indentured servants who would have worked at the mine in precarious conditions, another possibility is that the remains could be linked to the collapse of a reservoir in Potosi during the 1600s, which killed some 2,000 people, said Jose Antonio Fuertes, a historian at the national mint.

The Andean city, once among the world's biggest cities, now has a population of 200,000, according to AFP. Last month, UNESCO placed the city and the increasingly unstable Cerro Rico on its World Heritage in Danger list due to "uncontrolled mining operations."